The present invention relates to a lighting apparatus whose light source is made up of light emitting diodes.
In a conventional fluorescent lighting apparatus, as shown in FIG. 7, an alternating current (AC) voltage from a commercial power source 1 is converted into a direct current (DC) voltage by a diode bridge full-wave rectifier circuit 2, and the DC voltage is boosted by a booster circuit 3 and is supplied to a high-frequency inverter circuit 4. The high-frequency inverter circuit 4 converts the output voltage of the booster circuit 3 into a high-frequency voltage and applies the voltage to a fluorescent lamp serving as the light source.
In the booster circuit 3, a condenser 6 is connected between output terminals of the full-wave rectifier circuit 2, and a switch element 8 formed of a transistor or the like is connected in series with an inductor 8 between the output terminals of the full-wave rectifier circuit 2. Further, a series of a diode 9 and a smoothing condenser 10 of a large capacity is connected in parallel with the switch element 8 through a diode. The switch element 8 is turned on and off by a control circuit 11.
The inverter circuit 4 is a half bridge inverter in which a series circuit of switch elements 12 and 13 such as transistors or the like is connected between both ends of the smoothing condenser 10, a series circuit of condensers 14 and 15 is connected in parallel with the series circuit of switch elements 12 and 13, and a fluorescent lamp 5 is connected in series with a choke coil 16 between a junction between the switch elements 12 and 13 and a junction between the condensers 14 and 15. A pre-heating condenser 17 is connected between filament electrodes of the fluorescent lamp 5.
In the booster circuit 3, when the switch element 8 is turned on by the control circuit 11, a current flows into an inductor 7 and is stored therein as magnetic energy. Then, the switch element 8 is turned off by the control circuit 11. At this time, a current flows from the inductor 7 through the diode 9 to the smoothing condenser 10. This operation causes the magnetic energy stored in the inductor 7 to be converted into electrostatic energy and stored in the smoothing condenser 10. The smooth condenser 10 performs a smoothing in which the DC voltage supplied from the rectifier circuit 2 rendered substantially flat by absorbing a ripple component having a commercial frequency of 50 Hz.
In the inverter circuit 4, a rectangular waveform voltage is generated at the junction between the switch elements 12 and 13 by alternately turning on and off the switch elements 12 and 13, so that a pre-heating current initially flows to the filament electrodes of the fluorescent lamp 5 through the pre-heating condenser 17 and then the fluorescent lamp 5 starts lighting by a high voltage applied between the filament electrodes.
In this lighting apparatus, the high-frequency inverter circuit 4 is used to prevent flickering of the fluorescent lamp 5, and the booster circuit 3 is used to drive the inverter circuit 4. In addition, use of an alternating current prevents mercuric vapor from being displaced on one electrode side in the fluorescent lamp 5.
In a conventional light emitting diode lighting apparatus, as shown in FIG. 8, an AC voltage of the commercial power source 21 is converted into a DC voltage by a diode bridge full-wave rectifier circuit 22, and is smoothing by a smooth condenser 23. In addition, diode units are connected in series with respective current limiting resistors 24 between both ends of the smoothing condenser 23. Each diode unit is constituted by a plurality of light emitting diodes 25 which are connected in series with each other to form a light source.
Each current limiting resistor 24 limits a current flowing through a corresponding diode unit to a constant value, thereby causing the light emitting diodes 25 of the corresponding diode unit to light.
The conventional fluorescent lighting apparatus has the following problems. The number of circuit components is increased since the high-frequency inverter circuit and the booster circuit are required in order to convert an AC voltage of the commercial power source into a high-frequency voltage. The power loss is enlarged since electric power conversion such as the AC to DC voltage conversion and the DC to high-frequency voltage conversion is carried out for two times.
On the other hand, the conventional diode lighting apparatus has a problem that the power conversion efficiency is deteriorated by power loss at the current limiting resistors disposed in the current paths to the light emitting diodes. Hence, it can be considered that current limiting reactors are used instead of the current limiting resistors. However, since the current limiting reactors must have an inductance of about 100 mH to act on a commercial frequency of 50 Hz and are of a large size and a heavy weight, they are not suitable for practical use. Further, since a smoothing of a condenser-input type is performed on the commercial AC power source, there is a problem that the circuit components are adversely affected by harmonic components produced due to stationary periods present in the waveform of the input current.